The present invention relates to a method for enhancing recovery of the petroleum from an oil bearing formation. In particular, the invention relates to a method for introducing a surfactant into an oil bearing formation.
In the recovery of oils from reservoirs, the use of primary production techniques (i.e., use of only the initial formation energy to recover the crude oil), followed by the secondary technique of water flooding recovers only about 60% to 70% of the original oil present in the formation.
Moreover, the use of certain enhanced oil recovery (EOR) techniques is also known in the art. These techniques can be generally classified as a thermally based recovery method, i.e., utilizing steam, or a gas-drive method that can be operated under either miscible or non-miscible conditions. Typical gases employed in gas-drive method include those normally referred to as non-condensable gases, for example, nitrogen, carbon dioxide, methane, mixtures of methane with ethane, propane, butane, or higher hydrocarbon homologues.
In each of these EOR methods, it has been proposed to introduce a surfactant in order to increase the effectiveness of the particular process. For example, a number of commercial surfactants have been injected along with a steam stream to create a steam-foam flood. Surfactants form a foam that inhibits the flow of the steam into that portion of the formation containing only residual oil saturation in serves to physically block the volumes through which steam would otherwise shortcut. This forces the steam to remove the recoverable hydrocarbons from the lesser portions of the reservoir to the production well. Surfactants have also been employed within gas-drive methods in order to overcome a similar problem relating to bypassing of the non-condensable gas, e.g., CO.sub.2, through pores of the reservoir.
Typically, the surfactant is introduced according to a slug flow injection process in which the surfactant is continuously introduced over an eight hour period so as to generate a foam in the reservoir which foam formation can be evidenced by an increase the wellhead pressure from, e.g., 400 psi to 500 psi. The flow of surfactant is then stopped for the balance of the cycle, e.g., 16 hours in the traditional 24 hour cycle. Because many foams degrade relatively quickly, there can be substantially no foam present in the formation during much of this 24 hour cycle.
Thus, the need still exists for a method which makes more effective use of the surfactant during an EOR process.
Accordingly, it is an object of the present invention to provide a method which allows for a decrease in the amount of surfactant employed while at the same time maintaining foam in the formation.
These and further objects will be apparent from the specification and claims which follow.